Municipal and industrial wastewater treatment facilities often include primary, secondary and tertiary processes to treat wastewater to remove contaminants, such as suspended solids, biodegradable organics, phosphorus, nitrogen, microbiological contaminants, and the like, to provide a clean effluent. The clean effluent is typically subject to strict local, state and federal regulations.
The primary treatment processes often includes screens, grit chambers and/or primary clarifiers to remove large solids and other suspended matter to provide a primary effluent. Activated sludge is one type of secondary process which utilizes an aeration tank(s) which contains a large population of microorganisms that ingest contaminants in the primary effluent to form biological “flocs.” Oxygen is typically fed into the aeration tank(s) to promote growth of these biological flocs. The combination of primary effluent, or in some cases raw sewage, and biological flocs is commonly known as mixed liquor. The population or concentration of microorganisms in the mixed liquor is often referred to as mixed liquor suspended solids (MLSS).
After sufficient treatment in the aeration tank(s), the biological flocs in the mixed liquor are then typically sent to a secondary clarifier where the biological flocs are separated by gravity from the mixed liquor to provide a secondary effluent and a settled sludge. The secondary effluent, or “clean” effluent, may be discharged back to the environment or processed by additional tertiary treatment processes. The majority of the settled sludge in the secondary clarifier is typically recycled back to the aeration tank(s) by a return activated sludge subsystem. The remaining, excess sludge is wasted from the system to control the concentration of mixed liquor suspended solids.
However, separation of the biological flocs from the mixed liquor in the secondary clarifier is difficult because the biological flocs are only marginally heavier than water, and therefore settle very slowly. As a result, the secondary clarifier of a typical activated sludge process is the bottleneck in most wastewater treatment processes that utilize activated sludge as a secondary process. The crucial solids separation step of the biological flocs from the mixed liquor in the secondary clarifier is therefore typically the rate limiting process which is governed by a variety of factors, most notably the specific gravity, or density, of the biological flocs.
Moreover, solids separation in the secondary clarifier in a typical activated sludge processes is often unreliable due to the many types of settling problems that are caused by inter alia: overgrowth of filamentous organisms, viscous bulking caused by the overgrowth of either zoogleal organisms or exocellular polysaccharide material, pin floc, straggler floc, excessive solids loading on the secondary clarifiers, excessive secondary clarifier surface overflow rate, and the like.